Use beautifulsoup4 CSS selectors to simplify code and type checking

As the types-beautifulsoup4 package gets installed in the swh virtualenv as it is a swh-scanner test dependency, some mypy errors were reported related to beautifulsoup4 typing.

As the returned type for the find method of bs4 is the following union: Tag | NavigableString | None, isinstance calls must be used to ensure proper typing which is not great.

So prefer to use the select_one method instead where a simple None check must be done to ensure typing is correct as it is returning Optional[Tag]. In a similar manner, replace use of find_all method by select method.

It also has the advantage to simplify the code.